In the past year we have explored ways of directing liposomes to particular target cells or tissues. When hapten-modified human lymphocytes were incubated with hapten-modified liposomes and the corresponding bivalent antibody, liposomes bound in large numbers to the cell surface. However, contents of the bound vesicles did not enter the cells. When murine myeloma cells (MOPC 315, secreting an IgA anti-trinitrophenyl) were incubated with liposomes carrying a nitrophenyl hapten, again there was binding but no delivery of liposome contents into the cells. We are currently extending these studies to phagocytic cell lines. We have designed liposomes to release an entrapped drug preferentially at temperatures obtainable by mild local hyperthermia, for example in the treatment of tumors. This system makes use of increased release of liposome contents near their phase transition temperature. In the presence of serum the ratio of drug release at 43 degrees to that at 37 degrees can be made greater than 100:1. We have tested the approach in an in vitro cell system and are preparing now for in vivo studies.